Soft magnetic materials are used for applications, such as core materials in inductors, stators and rotors for electrical machines, actuators, sensors and transformer cores. Traditionally, soft magnetic cores, such as rotors and stators in electric machines, are made of stacked steel laminates. Soft Magnetic Composite (SMC) materials are based on soft magnetic particles, usually iron-based, with an electrically insulating coating on each particle. The SMC components are obtained by compacting the insulated particles using a traditional powder metallurgical (PM) compaction process, optionally together with lubricants and/or binders. By using the powder metallurgical technique it is possible to produce materials having higher degree of freedom in the design of the SMC component than by using the steel laminates, as the SMC material can carry a three dimensional magnetic flux, and as three dimensional shapes can be obtained by the compaction process.
Two key characteristics of an iron core component are its magnetic permeability and core loss characteristics. The magnetic permeability of a material is an indication of its ability to become magnetised or its ability to carry a magnetic flux. Permeability is defined as the ratio of the induced magnetic flux to the magnetising force or field intensity. When a magnetic material is exposed to a varying field, energy losses occur due to both hysteresis losses and eddy current losses. The hysteresis loss (DC-loss), which constitutes the majority of the total core losses in most motor applications, is brought about by the necessary expenditure of energy to overcome the retained magnetic forces within the iron core component. The forces can be minimized by improving the base powder purity and quality, but most importantly by increasing the temperature and/or time of the heat treatment (i.e. stress release) of the component. The eddy current loss (AC-loss) is brought about by the production of electric currents in the iron core component due to the changing flux caused by alternating current (AC) conditions. A high electrical resistivity of the component is desirable in order to minimise the eddy currents. The level of electrical resistivity that is required to minimize the AC losses is dependent on the type of application (operating frequency) and the component size.
The hysteresis loss is proportional to the frequency of the alternating electrical fields, whereas the eddy current loss is porportional to the square of the frequency. Thus, at high frequencies, the eddy current loss matters mostly and it is especially required to reduce the eddy current loss and still maintaining a low level of hysteresis loss. For applications operating at high frequencies where insulated soft magnetic powders are used it is desirable to use powders having finer particle size, as the eddy currents created can be restricted to a smaller volume provided the electrical insulation of the individual powder particles is sufficient (inner-particle Eddy currents). Thus, fine powders as well as high electrical resistivity will become more important for components working at high frequency. Independent on how well the particle insulation works there is always a part of unrestricted Eddy currents within the bulk of the component, causing loss. The bulk Eddy-current loss is proportional to the cross sectional area of the compacted part that carries magnetic flux. Thus, components having large cross sectional area that carry magnetic flux will require higher electrical resistivity in order to restrict the bulk Eddy current losses.
Insulated iron-based soft magnetic powder having an average particle size of 100-400 μm, e.g. between about 180 μm and 250 μm and less than 10% of the particles having a particle size below 45 μm (40 mesh powder) are normally used for components working at a frequency up to 1 kHz. Powders having an average particle size of 50-150 μm, e.g. between about 80 μm and 120 μm and 10-30% less than 45 μm (100 mesh powder) may be used for components working from 200 Hz up to 10 kHz, whereas components working at frequencies from 2 kHz up to 50 kHz are normally based on insulated soft magnetic powders having an average particle size about 20-75 μm, e.g. between about 30 μm and 50 μm and more than 50% is less than 45 μm (200 mesh powder). The average particle size and particle size distribution should preferably be optimized according to the requirements of the application. Thus examples of weight average particle sizes are 10-450 μm, 20-400 μm, 20-350 μm, 30-350 μm, 30-300 μm, 20-80 μm, 30-50 μm, 50-150 μm, 80-120 μm, 100-400 μm, 150-350 μm, 180-250 μm, 120-200 μm.
Research in the powder-metallurgical manufacture of magnetic core components using coated iron-based powders has been directed to the development of iron powder compositions that enhance certain physical and magnetic properties without detrimentally affecting other properties of the final component. Desired component properties include e.g. a high permeability through an extended frequency range, low core losses, high saturation induction, and high mechanical strength. The desired powder properties further include suitability for compression moulding techniques, which means that the powder can be easily moulded to a high density component, which can be easily ejected from the moulding equipment without damages on the component surface.
Examples of published patents are outlined below.
U.S. Pat. No. 6,309,748 to Lashmore describes a ferromagnetic powder having a diameter size of from about 40 to about 600 microns and a coating of inorganic oxides disposed on each particle.
U.S. Pat. No. 6,348,265 to Jansson teaches an iron powder coated with a thin phosphorous and oxygen containing coating, the coated powder being suitable for compaction into soft magnetic cores which may be heat treated.
U.S. Pat. No. 4,601,765 to Soileau teaches a compacted iron core which utilizes iron powder which first is coated with a film of an alkali metal silicate and then over-coated with a silicone resin polymer.
U.S. Pat. No. 6,149,704 to Moro describes a ferromagnetic powder electrically insulated with a coating of a phenol resin and/or silicone resin and optionally a sol of titanium oxide or zirconium oxide. The obtained powder is mixed with a metal stearate lubricant and compacted into a dust core.
U.S. Pat. No. 7,235,208 to Moro teaches a dust core made of ferromagnetic powder having an insulating binder in which the ferromagnetic powder is dispersed, wherein the insulating binder comprises a trifunctional alkyl-phenyl silicone resin and optionally an inorganic oxide, carbide or nitride.
Further documents within the field of soft-magnetics are Japanese patent application JP 2005-322489, having the publication number JP 2007-129154, to Yuuichi; Japanese patent application JP 2005-274124, having the publication number JP 2007-088156, to Maeda; Japanese patent application JP 2004-203969, having the publication no JP 2006-0244869, to Masaki; Japanese patent application 2005-051149, having the publication no 2006-233295, to Ueda and Japanese patent application 2005-057193, having the publication no 2006-245183, to Watanabe.